Support emitting Makefile-syntax depfiles like gcc/clang/rustc.

[anp]

The motivation here is being able to use bindgen during a ninja build without losing dependency information.

I'm not particularly happy with the approach I've taken of wrapping ParseCallbacks. The main alternative I see is to have the parse callbacks separated into a concrete type and a trait object, and to always collect filenames in the struct. I'm open to suggestions for better ways to collect the needed metadata, though.

[emilio]

So the goal of this PR seems worth it, but I believe this could be much simpler, I'm not a fan of the way this has to wrap the parsing callbacks to work. Is there any reason this actually needs to use the parse callbacks mechanism at all?

It seems it only cares about include_file() (and the options), right? It should be straight-forward to implement it using something like this, in a way that involves no breaking changes, or am I missing something?

diff --git a/src/ir/context.rs b/src/ir/context.rs
index ccb05e75..8b3da71d 100644
--- a/src/ir/context.rs
+++ b/src/ir/context.rs
@@ -465,6 +465,10 @@ pub struct BindgenContext {
     /// Populated when we enter codegen by `compute_has_float`; always `None`
     /// before that and `Some` after.
     has_float: Option<HashSet<ItemId>>,
+
+    /// The file dependencies for the bindings, if we're using the `depfile`
+    /// option.
+    deps: BTreeSet<String>,
 }
 
 /// A traversal of allowlisted items.
@@ -579,6 +583,8 @@ If you encounter an error missing from this list, please file an issue or a PR!"
             have_destructor: None,
             has_type_param_in_array: None,
             has_float: None,
+            // Or maybe account for the options here instead of using default?
+            deps: Default::default(),
         }
     }
 
@@ -632,6 +638,16 @@ If you encounter an error missing from this list, please file an issue or a PR!"
         self.options().parse_callbacks.as_ref().map(|t| &**t)
     }
 
+    /// Called on header inclusion.
+    pub fn include_file(&mut self, filename: String) {
+        if let Some(cbs) = self.parse_callbacks() {
+            cbs.include_file(&filename);
+        }
+        if self.options.depfile.is_some() {
+            self.deps.insert(filename);
+        }
+    }
+
     /// Define a new item.
     ///
     /// This inserts it into the internal items set, and its type into the
diff --git a/src/ir/item.rs b/src/ir/item.rs
index 45415045..d7c92ab4 100644
--- a/src/ir/item.rs
+++ b/src/ir/item.rs
@@ -1415,9 +1415,7 @@ impl ClangItemParser for Item {
                             );
                         }
                         Some(filename) => {
-                            if let Some(cb) = ctx.parse_callbacks() {
-                                cb.include_file(&filename)
-                            }
+                            ctx.include_file(filename);
                         }

[anp]

Yeah, that approach is much cleaner, thanks! Addressed comment and have a question on testing (see review comment).

[emilio]

Thanks, this looks really close!

[anp]

OK, I think that's all the review feedback and my TODO resolved. This should be good for another pass.

[emilio]

Looks great, thank you!